TUESDAY, June 1 (HealthDay News) -- Low levels of physical activity and fitness significantly increase the risk of developing high blood pressure, a 20-year study of young adults shows.

The study, released online June 1 in advance of publication in the July print issue of the journal Hypertension, found that about one-third of all high blood pressure cases could be prevented with increased physical fitness.

"Those who were the least physically fit, as determined by the amount of time on a treadmill and self-report, were more likely to develop hypertension," said study author Mercedes Carnethon, an assistant professor of preventive medicine at the Northwestern University Feinberg School of Medicine in Chicago.

As many as one in three Americans has high blood pressure, also called hypertension, according to the most recent estimates from the U.S. Centers for Disease Control and Prevention. High blood pressure is a risk factor for heart disease, stroke and kidney disease, according to the CDC.

Regular physical activity is one way that you can help keep your blood pressure at normal levels, and the CDC recommends at least 30 minutes of moderate physical activity on most days of the week.

But, researchers wondered, could your levels of physical activity and fitness as a young adult affect your risk of developing high blood pressure later in life?

To answer that question, 4,618 men and women between 18 and 30 years old were recruited for a long-term study of cardiovascular disease risk factors. Study volunteers completed a treadmill test and a physical activity questionnaire when the study began. In addition, their overall health was assessed at six follow-up appointments over 20 years.

Just over 1,000 people in the study developed high blood pressure, which was defined as having blood pressure that's higher than 140/80 mm Hg or having been prescribed high blood pressure medication.

Even after adjusting for other known heart disease and high blood pressure risk factors -- such as smoking, age, race, sex, cholesterol and diet -- the researchers found that lower levels of physical activity and fitness were associated with an increased risk of high blood pressure.

Carnethon said this study is especially helpful because it didn't rely solely on self-report of physical activity, but relied on an objective measure of physical activity -- the treadmill test.

If people moved more and were able to increase their fitness level, the researchers estimate that about 34% of hypertension cases could be prevented.

Carnethon said that the reason sedentary behavior in young adults translated to a higher risk of elevated blood pressure later in life was probably because the sedentary behavior didn't change as people grew older.

"What we find is that those health behaviors tend to track to older ages," said Carnethon.

"The results of this study aren't too surprising, but what I think is impressive is the amount of hypertension that can be prevented. For example, in white women, the difference between low levels of fitness and high levels of fitness is a fivefold higher risk in the low level group. The magnitude of the difference was surprising," said Dr. Goutham Rao, clinical director of the Weight Management and Wellness Center at Children's Hospital of Pittsburgh.

"Hypertension is largely controllable or reversible. The good news is that there's no point at which you can not benefit from increased activity," said Rao. But, he said, it's better to start younger because people who are active in their youth are more likely to stay active as adults.